1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for transmitting and receiving ultrasonic waves to generate an ultrasonic diagnostic image and a reception focusing processing method to be used in the ultrasonic diagnostic apparatus.
2. Description of a Related Art
In medical fields, various imaging technologies have been developed for observation and diagnoses within an object to be inspected. Especially, ultrasonic imaging for acquiring interior information of the object by transmitting and receiving ultrasonic waves enables image observation in real time and provides no exposure to radiation unlike other medical image technologies such as X-ray photography or RI (radio isotope) scintillation camera. Accordingly, ultrasonic imaging is utilized as an imaging technology at a high level of safety in a wide range of departments including not only the fetal diagnosis in obstetrics, but gynecology, circulatory system, digestive system, and so on.
The principle of ultrasonic imaging is as follows. Ultrasonic waves are reflected at a boundary between regions having different acoustic impedances like a boundary between structures within the object. Therefore, by transmitting ultrasonic beams into the object such as a human body and receiving ultrasonic echoes generated within the object, and obtaining reflection points, where the ultrasonic echoes are generated, and reflection intensity, outlines of structures (e.g., internal organs, diseased tissues, and so on) existing within the object can be extracted.
Generally, in an ultrasonic diagnostic apparatus, an ultrasonic probe including plural ultrasonic transducers (vibrators) having transmitting and receiving functions of ultrasonic waves is used. Reception signals outputted from the vibrators that have received ultrasonic echoes have delays according to differences of distances between the focal point of ultrasonic waves and the respective vibrators. Accordingly, by providing delays according to the positions of the vibrators to those reception signals and adding those reception signals to one another, beam forming processing (reception focusing processing) of forming a focal point in a specific position is performed.
In a system of analog beam forming, a delay time can be set according to a pitch of a tap of an analog delay line (delay element) in steps of several tens of nanoseconds. On the other hand, in a system of digital beam forming, basically, a delay time depends on clock fineness in analog/digital conversion. For example, when sampling at 50 MHz is performed, the delay time can be set in steps of 20 ns.
The pitch in the amounts of delay may cause so-called quantization sidelobes, and therefore, efforts are made for a finer pitch. For example, data in locations between adjacent two sampling points are generated by interpolation, or data in locations between adjacent two sampling points are generated by inserting zero values into data (actual data) obtained by reception of ultrasonic echoes, and then performing low-pass filter processing thereon.
As a related technology, Japanese Patent Application Publication JP-A-7-303638 discloses a multi-channel digital receiving apparatus for acquiring in-phase components and orthogonal components from signals respectively reaching plural channels from one signal source through different transfer pathways, by digital processing. The receiving apparatus includes plural channels of receiving means for respectively receiving signals reaching from one signal source through different transfer pathways to output analog reception signals, plural channels of A/D converting means for converting the respective analog reception signals into digital data, a memory for storing the digital data, writing control means for sampling the digital data at a predetermined sampling interval ΔT to write the digital data in the memory, readout control means for reading out two or more pieces of digital data having sampling times near time tm shifted from certain target time t0 by a time period Tm of the integral multiple of the sampling interval ΔT, interpolation computing means for computing interpolated digital data at time tk shifted from time tm by a time period τk smaller than the sampling interval ΔT by interpolation computation using the two or more pieces of digital data read out from the memory, sign inverting means for inverting the sign of the interpolated digital data, a switching selecting means for selecting the interpolated digital data, the digital data with the inverted sign, or “0” according to the target time to, low-pass filter means for extracting only basebands and outputting the basebands as channel in-phase components or channel orthogonal components, in-phase component adding means for adding in-phase components of the respective channels to acquire a synthesized in-phase component, and orthogonal component adding means for adding orthogonal components of the respective channels to acquire a synthesized orthogonal component.
FIG. 8 is a waveform chart for explanation of sampling and data delay in conventional beam forming. According to the conventional method, ultrasonic reception signals are phase-matched and added to one another in a form of RF signals. In digital beam forming, delaying of data is performed by adjusting the readout timings of data stored in a memory. However, the data stored in the memory exist at a time interval of a sampling period, and are coarse for setting amounts of delay. Generally, when coarse amounts of delay are set, so-called quantization sidelobes are generated, and image quality becomes deteriorated because the obtained image contains artifacts.
Accordingly, as shown in FIG. 9, it is required to set a finer amount of delay than the sampling period. Here, FIG. 9(a) shows original data, and FIG. 9(b) shows data delayed by a time period “t”. As a method of interpolating data between actual data, there are methods of linearly interpolating data between adjacent two pieces of actual data as shown in FIG. 10, interpolating data by using a spline function as shown in FIG. 11, and so on. Further, because of the simple circuit configuration, as shown in FIG. 12, a method of generating interpolated data by inserting zero data between actual data and performing low-pass filter processing thereon. Here, FIG. 12(a) shows a state in which the zero data has been inserted, and FIG. 12(b) shows a state in which the low-pass filter processing has been performed.
On the other hand, it is also possible that the reception signals (RF signals) are orthogonally detected and complex baseband signals (I-signals and Q-signals) are generated, and then, the complex baseband signals are provided with delays for phase-matching and added to one another. For just performing orthogonal detection, conditions are the same as those of phase-matching and addition of the above-mentioned RF signals. After the orthogonal detection, the signal band is narrow, and resampling can be performed at a sampling frequency equal to or more than twice the signal band. That is, by resampling at a slow sampling clock of about a fraction of the sampling clock of the original RF signals, the number of data can be reduced.
However, at the same time, the sampling period of data becomes coarser. Accordingly, in the case of generating the finer amount of delay by the above-mentioned interpolation processing, several times of data interpolation processing of the RF signals is required as shown in FIG. 13.